1. Field of the Invention
The present invention relates to a semiconductor device including an integrated circuit using thin film transistors on a substrate and a method of fabricating the same. Particularly the invention relates to a structure of, for example, an electro-optical device typified by a liquid crystal display device and an electronic equipment incorporating the electro-optical device.
2. Description of the Related Art
Development has been made on a semiconductor device typified by an active matrix type liquid crystal display device in which a number of TFTs (thin film transistors) are arranged on a substrate. The TFT has a laminate structure including at least an active layer made of an island-like semiconductor film, a first insulating layer provided at a substrate side of the active layer, and a second insulating layer provided at a side opposite to the substrate side of the active layer. Alternatively, the TFT has a laminate structure including an active layer and a second insulating layer provided to be in close contact with a surface of the active layer at a side opposite to a substrate side thereof, in which the first insulating layer is omitted.
The structure in which a gate electrode is provided so as to apply a predetermined voltage to the active layer through the first insulating layer is called an inverted stagger type or a bottom gate type. On the other hand, the structure in which a gate electrode is provided so as to apply a predetermined voltage to the active layer through the second insulating layer is called a forward stagger type or top gate type.
It has been considered that a crystalline semiconductor capable of obtaining high mobility in addition to an amorphous semiconductor is suitable for a semiconductor film used for a TFT. Here, the crystalline semiconductor includes a single crystal semiconductor, a polycrystal semiconductor, and a microcrystal semiconductor. The insulating layer is typically formed of a material such as silicon oxide, silicon nitride, or silicon nitride oxide.
It is known, as the semiconductor film above, a semiconductor disclosed in Japanese Patent Application Laid Open No. Hei. 7-130652, No. Hei. 8-78329, No. Hei. 10-135468, or No. Hei. 10-135469.
It has been known that a thin film of the above material fabricated by a well-known film forming technique, such as a CVD (Chemical Vapor Deposition), a sputtering method, and a vacuum evaporation method, includes internal stress. The internal stress has been classified into intrinsic stress which the thin film intrinsically has, and thermal stress due to a difference in thermal expansion coefficient between the thin film and the substrate. It has been possible to neglect the thermal stress by controlling the thermal expansion coefficient of the substrate and process temperature of fabricating steps of the TFT. However, the generation mechanism of the intrinsic stress has not been necessarily clarified, and it has been considered that the intrinsic stress is generated by a complicated combination of a phase change and composition change of the thin film during a growth process thereof, by heat treatment thereafter, and the like.
In general, as shown in FIG. 3A, when a thin film is contracted with respect to a substrate, the substrate is deformed by the influence while the thin film is located inside. Thus, the internal stress is called tensile stress. On the other hand, as shown in FIG. 3B, when the thin film is expanded, the substrate is compressed and is deformed while the thin film is located outside. Thus, the internal stress is called compressive stress. Like this, the definition of the internal stress has been considered while the substrate is made the center. Also in this specification, the internal stress is set forth in accordance with this definition.
It has been known that volume contraction occurs during a process of crystallization in a crystalline semiconductor film fabricated from an amorphous semiconductor film by a thermal annealing method or a laser annealing method. Although depending on the state of the amorphous semiconductor film, it has been considered that the rate is about 0.1 to 10%. As a result, there has been a case where the tensile stress is generated in the crystalline semiconductor film and its intensity becomes about 1xc3x97109 Pa. Besides, it has been known that the internal stress of an insulating film, such as a silicon oxide film, a silicon nitride film, or a silicon nitride oxide film, is variously changed from the compressive stress to the tensile stress by fabricating conditions and subsequent heat treatment conditions.
In the technical field of a VLSI, a problem of stress has been pointed out as one of causes of a poor device. With the improvement in integration, it has inevitably become impossible to neglect an influence of local stress. For example, it has been considered that a heavy metal impurity is captured in a region where the stress is concentrated so that various poor modes are caused, or dislocation generated to relieve the stress is also a factor to deteriorate the characteristics of a device.
However, with respect to a TFT formed by laminating a plurality of thin films, such as a semiconductor film and an insulating film, an influence caused by the interaction between the respective internal stresses of the thin films has not been sufficiently clarified.
Although there are some characteristic parameters expressing TFT characteristics, an electric field mobility is regarded as one standard indicating the level of performance. In order to realize a high field effect mobility, the structure of a TFT and its fabricating process have been carefully studied in view of theoretical analysis and empirical side. As especially important factors, it has been considered that it is necessary to decrease a bulk defect density in a semiconductor layer and an interface level density at an interface between a semiconductor layer and an insulating layer to the utmost degree.
In order to decrease the bulk defect density and interface defect density formed in a crystalline semiconductor layer, the present inventor has considered it to be a problem that the defect density is decreased while internal stresses of respective thin films are taken into consideration and a stress balance is taken, in addition to optimization of fabricating conditions of a TFT.
An object of the present invention is to solve the foregoing problem and to realize a TFT in which bulk defect density and interface defect density are decreased while distortion is not generated in a crystalline semiconductor layer.
As described in the Background of the Invention section, tensile stress is inherent in a crystalline semiconductor film fabricated from an amorphous semiconductor film. In a TFT including an active layer of such a crystalline semiconductor film, it has been necessary to consider a stress balance in order to laminate a gate insulating film, other insulating films and conductive films without generating distortion to the crystalline semiconductor film.
The stress balance to be considered here is not such that composed stress is made zero by compensating the internal stresses of the laminated thin films, but such that the thin films having inherent internal stresses are laminated, with the crystalline semiconductor film including tensile stress as the center, in such a direction that distortion does not occur in the crystalline semiconductor film.
FIGS. 4A and 4B are views for explaining the concept of the present invention. With respect to a crystalline semiconductor film having tensile stress, the present inventor has considered to be desirable that a thin film provided at a substrate side of the crystalline semiconductor film has tensile stress (FIG. 4B). On the other hand, the present inventor has considered to be desirable that a thin film provided on a surface of the crystalline semiconductor film at a side opposite to the substrate side has compressive stress (FIG. 4A). In all events, when the crystalline semiconductor film is contracted, if a stress acts in a direction to expand this, it is expected that distortion occurs in crystal grain boundaries and micro cracks are formed. In such a case, dislocations and crystal defects are produced in the region, and a number of unpaired bonds are formed. Thus, when the thin film provided at the substrate side of the crystalline semiconductor film is made to have the tensile stress, the stress can be given in the same direction as the direction in which the crystalline semiconductor layer is contracted. On the contrary, when the thin film provided at the side opposite to the substrate side with respect to the crystalline semiconductor film is made to have the compressive stress, the stress can be given in the same direction as the direction in which the crystalline semiconductor layer is contracted. That is, when such a structure is adopted that stress is given from other thin films in the direction to contract the crystalline semiconductor film, the defect density can be effectively decreased.
For the purpose of controlling the internal stress of the thin film, it was sufficient if fabricating conditions and subsequent heat treatment conditions were considered. For example, in a silicon nitride oxide film fabricated by a plasma CVD method, it was possible to change the stress from the compressive stress to the tensile stress by changing the composition ratio of nitrogen and oxygen or the hydrogen content. In a silicon nitride film fabricated by a plasma CVD method, it was possible to change the intensity of the internal stress by changing a film deposition rate.
Further, the important point in consideration of the stress balance was temperature control in all fabricating steps of a TFT. In a thin film fabricated by a plasma CVD method or a sputtering method, even if the film had a predetermined internal stress in the initial state, there was a case where the stress was changed to quite the opposite direction internal stress by a substrate heating temperature in a subsequent step. On the contrary, it was also possible to change the internal stress by using this property. For example, when a heat treatment at a temperature of 300xc2x0 C. or more was applied to a silicon nitride film having compressive stress, it was also possible to change the stress to tensile stress.
When a gate electrode was provided to apply a predetermined voltage through a first insulating layer provided at a substrate side of an active layer made of an island-like semiconductor film formed on a substrate, it was possible to form an inverted stagger type or bottom gate type TFT. When a gate electrode is provided to apply a predetermined voltage to an active layer through a second insulating layer provided at a side opposite to a substrate side of the active layer, it was possible to form a forward stagger type or top gate type TFT.
Although a material of an insulating film used for the first insulating layer or the second insulating layer was not particularly limited, it was necessary to be able to control the internal stress in some way. For that purpose, a silicon nitride film, a silicon nitride oxide film, a silicon oxide film, a tantalum oxide film, and the like were suitable. Although a method of fabricating the silicon nitride film is not limited, for example, in the case where the film is formed by a plasma CVD method, the film can be formed from a mixture gas of SiH4, NH3, N2, and H2. By changing a mixture ratio of the gas and discharge power density, it was possible to fabricate the silicon nitride film under conditions of different film formation rates. As a measuring device of the internal stress, Model-30114 made by Ionic System Inc. was used. A sample fabricated on a silicon wafer was used for measurement.
With respect to values of the internal stress, it is assume that the tensile stress is indicated by a positive value and the compressive stress is indicated by a negative value so that distinction can be made. According to data of FIG. 17, although any of silicon nitride films fabricated at a substrate temperature of 400xc2x0 C. and at different film deposition rates had compressive stress, when a heat treatment at 500xc2x0 C. for 1 hour was applied, it was possible to change the compressive stress to the tensile stress. Such change was realized when a heat treatment at a temperature higher than a substrate temperature at film formation was carried out, and it was considered that densification of the silicon nitride film was the cause. Thus, it was possible to fabricate both of a film having the compressive stress and a film having the tensile stress for the silicon nitride film.
A silicon nitride oxide film was fabricated from a mixture gas of SiH4 and N2O using a plasma CVD method. Also in this case, it was possible to fabricate the silicon nitride oxide film by changing the mixture ratio of gas or discharge power density to make film deposition rate different. FIG. 18 shows values of the internal stress of the silicon nitride oxide films fabricated at a substrate temperature of 400xc2x0 C. Any of respective samples with different film deposition rates had compressive stress. Even if a heat treatment at 450xc2x0 C. for 4 hours was further applied, the state was unchanged though the absolute value of the compressive stress became small.
Although FIG. 19 similarly shows data of internal stress of silicon nitride oxide films, this drawing shows data of silicon nitride oxide films fabricated by further mixing NH3 to SiH4 and N2O. When the NH3 gas was added at film formation, the characteristic was changed from the compressive stress to the side of the tensile stress. Further, when a heat treatment at 550xc2x0 C. for 4 hours was applied to the samples, it was possible to increase the tensile stress. The change like this corresponded to the change of composition ratio of a nitrogen content and an oxygen content in the silicon nitride oxide film. Table 1 shows the result of measurement of the content of each element in the silicon nitride oxide film measured by Rutherford backscattering method (RBS).
When the nitrogen content and the oxygen content in a silicon nitride oxide film were 7 atomic % and 59.5 atomic %, respectively, it was possible to make the nitrogen content and the oxygen content 24.0 atomic % and 26.5 atomic %, respectively, by adding the NH3 gas of 30 SCCM at film formation. Besides, it was possible to make the nitrogen content and the oxygen content 44.1 atomic % and 6.0 atomic %, respectively, by adding the NH3 gas of 100 SCCM. That is, by adding the NH3 gas, it was possible to increase the nitrogen content in the silicon nitride oxide film and to decrease the oxygen content. At this time, it was possible to change the compressive stress to the tensile stress. When the composition of various silicon nitride oxide films obtained by adding the NH3 gas were investigated, in any film, the composition was such that the silicon content was about 34 atomic %, the hydrogen content was about 16 atomic %, and the sum of nitrogen and oxygen content was about 50 atomic %. The films having the nitrogen content of not less than 25 atomic % and less than 50 atomic % obviously exhibited the tensile stress, and the films having the nitrogen content of not less than 5 atomic % and less than 25 atomic % exhibited the compressive stress. It was possible to consider the change of the internal stress by heat treatment while relating it to the change of the hydrogen content in the film as shown in FIG. 20. The data of FIG. 20 show the result of measurement by FT-IR to the hydrogen content in the silicon nitride oxide films fabricated by adding the NH3 gas. By a heat treatment at 500xc2x0 C. for 1 hour, hydrogen bonded with silicon is first released. This tendency becomes remarkable as the substrate temperature (see Tsub expressed at the upper right of each graph of FIG. 20) at film formation becomes low. It is expected that when hydrogen bonded with silicon is released, unpaired bonds are produced, and the tensile stress is strengthened by the interaction (attractive force) of the unpaired bonds. Like this, it was also possible to change the internal stress by decreasing the hydrogen content in the film.
Like this, by controlling the film formation rate, by applying the heat treatment at a temperature higher than a substrate temperature of film formation, or by controlling film formation conditions, it was possible to control the internal stress. As is well known, a TFT is completed by repeating thin film formation and an etching process, and the important point here was the control of process temperature over all the fabricating steps. It was sufficient if the highest temperature of the process was determined in view of the internal stresses of thin films to be laminated.
A semiconductor device of the present invention comprises an active layer of an island-like semiconductor film formed over a substrate; a first insulating layer provided at a substrate side of the active layer and including a first silicon nitride oxide film having a nitrogen content higher than an oxygen content and a second silicon nitride oxide film having a nitrogen content lower than an oxygen content; and a second insulating layer provided to be in contact with a surface of the active layer at a side opposite to the substrate side and including a plurality of third silicon nitride oxide films each having a nitrogen content lower than an oxygen content.
In the semiconductor device of the present invention, the active layer has tensile stress, the first silicon nitride oxide film of the first insulating layer in which the nitrogen content is higher than the oxygen content has tensile stress, and each of the plurality of third silicon nitride oxide films of the second insulating layer in which the nitrogen content is lower than the oxygen content has compressive stress. It is desirable that a difference in absolute values of the tensile stresses between the first insulating layer and the semiconductor layer, or a difference in absolute values between the compressive stress of the second insulating layer and the tensile stress of the semiconductor layer is within 5xc3x97108 Pa.
Besides, in the semiconductor device of the present invention, the nitrogen content of the first silicon nitride oxide film in which the nitrogen content is higher than the oxygen content is not less than 25 atomic % and less than 50 atomic %, and the nitrogen content of each of the plurality of third silicon nitride oxide films in which the nitrogen content is lower than the oxygen content is not less than 5 atomic % and less than 25 atomic %.